Abstract EOLINK have developed an innovative floating wind turbine in which the single tower is replaced by a set of legs providing a pyramidal architecture. A 1/10 th scale prototype of EOLINK’s 12MW concept has been connected to the grid in April 2018 in France. Firstly, the paper describes the technical specifications of this device. Both the turbine and the floater have been designed using Froude scaling, in order to properly represent the EOLINK full scale 12MW concept. The device has been devised from scratch and deploys a Permanent Magnet Synchronous Generator (PMSG) and an individual electric blade pitch system. The patented mooring system comprises a single point mooring (SPM) system able to withstand very high tide ranges in shallow waters. Regarding monitoring, motions have been recorded using both an Inertial Measurement Unit (IMU) and high precision Global Positioning System (GPS) sensors. Mooring lines tensions have also been monitored. Wind is recorded using both an embedded anemometer on the floating turbine and onshore anemometers installed by IFREMER. This Institute has also measured wave height using a wave recorder. Secondly, experimental results during production and storm events are presented. The encountered environmental conditions highlight the capability of the EOLINK design to withstand harsh wind events, and its ability to produce 12MW using a small sized semi-submersible floater. Then, numerical analysis using FAST and Flexcom is compared with experimental results. Static analysis, decay-tests, Response Amplitude Operators (RAOs) and Power Spectral Densities (PSDs) results are detailed. Power production and the embedded control command capabilities are also presented.

Abstract Continually increasing demands on aquaculture products drive the current monoculture to upgrade and upscale because of not only economic but environmental sustainability reasons. Over the past decade, open water integrated multi-trophic aquaculture (IMTA) practiced as a potential alternative has been demonstratively illustrated from both scientific and public attention. Basing on previous studies of this synergistic aquaculture system, we, here, studied the physical environment in Onagawa Bay as the cornerstone for further IMTA implementation. Onagawa Bay locates in Miyagi Prefecture, Japan, and because of its mature practice on polyculture, it is recognized as a suitable site for IMTA. Unfortunately, the earthquake and tsunami in 2011 caused a huge uncertainty on physical environment changes. Still insufficient researches have been conducted on physical environment study, especially through modelling method. Here, adopting the three-dimensional Marine Environmental Committee (MEC) ocean model, we described the setup and validation for Onagawa Bay in this research. At the present stage, simulation results can best fit observation data on the tidal elevation with the correlation coefficient between observed and simulated tidal elevation reaching 0.96, captured the main characteristic of flow velocity, and exhibited homogenous tendency towards water temperature. Furthermore, through the plot of the residual velocity field and statistical seasonal velocity distribution, potential aquaculture configuration has been discussed spatio-temporally on the hypothesis that high current speeds contribute to the further implementation.

Abstract In 2011, the National Diet of Japan passed a law to implement the policy of “International Container Strategy Ports,” which would be applied to public-built private-management system and private company financing methods to container terminals at the main ports operated by Port Management Bodies which are local governments. It would also establish the port management corporation at each port in order to improve the productivity of container terminals throughout Japan. Port management corporations have already been established at several ports such as “Kobe-Osaka,” “Yokohama-Kawasaki” and “Nagoya-Yokkaichi.” The purpose of establishing these port management corporations for container terminals is to amend the rigid port management system by local governments, and to be prepared to provide a quick response to global economic changes and the wills of cargo owners and shipping companies. Current port management corporations, however, are facing various problems of management. On the other hand, as in the cases of the tidal inundation disaster at Kansai Airport and the port area due to heavy rain in 2018, port management needs to make new capital investment to cope with the issues such as the construction to protect ground structure against high tide and tsunamis. Therefore, the author would like to first analyze the issues that current port management face, and then propose solutions and possible scenarios for improving the productivity and sustainability of port management against high tide and tsunamis.

Abstract Receding seasonal sea ice extent over the Arctic Ocean is increasing access to what was a largely inaccessible region. At lower latitudes the complex vertical current structure associated with large amplitude, high frequency non-linear internal waves, sometimes referred to as solitons, present a significant challenge to the safe engineering design and operation of offshore infrastructure. In this paper we examine the prevalence this type of internal wave in the Arctic Ocean. To do so we will draw on both in situ and remotely sensed oceanographic data. This will be combined with state-of-the-art numerical modelling to demonstrate a link between the geographical occurrence of these waves and the tide. Whilst the link implies that these features are geographically limited, it is also likely that the geographical limits will change with declining sea ice cover. These results will then be used to provide a road map towards a methodology for forecasting the prevalence of these phenomena in a future Arctic Ocean.

Abstract Maintaining and retaining a quality sandy beach is a primary requirement for attracting people and tourists in any coastal country. Tourism Enterprises Company (TEC) in Kuwait owns 230 m long sandy beach in Ras Al-Ardh Sea Club, Salmiya, Kuwait. The beach has been eroding because of strong hydrodynamics forces from waves and currents. TEC wants to develop a stable sandy beach of 30 m wide. Kuwait Institute for Scientific Research (KISR), Kuwait is assigned to carry out the required scientific studies. In order to make sure a stable quality beach will exist, KISR has carried out the needed studies, which involves the field measurements such as bbathymetry survey, current and tidal variations, physical characteristics of beach soil, beach and sea bed profile, establishing the design parameters such as waves, currents, tide and wind. Hydrodynamic model study using DELFT3D model for the present and for the proposed extended groin conditions with beach nourishment were carried out. Also numerical modeling using GENESIS model to understand the future shore line changes due to the proposed development was carried out. Design of Groins to estimate the weight of armor units and weight of inner layers were carried out. The particle size and quantity of sand needed for reclamation of 30 m wide beach was estimated. Based on the study, it is recommended that the sandy soil to be used for 30 m wide beach nourishment should have D 50 greater than 0.42 mm (say 0.5 mm) and D 10 greater than 0.25 mm. The borrow pit much be selected by keeping this soil characters in mind. It is recommended to use a submerged offshore breakwater in order to retain the beach sand in place and for reducing the maintenance nourishment. Otherwise, large quantity of the capital nourished beach sand will escape into the deeper water due to strong current coupled with waves and steep seabed slopes. Environmental Impact Study was carried out as per Kuwait Environment Public Authority requirements to bring out the impacts due to beach filling and the construction submerged offshore barrier and extension of east groin for a distance of 30 m. TEC will implement the recommendations for developing the beach in Ras Al-Ardh sea club and will be useful to attract more people to use this beach.

Abstract A spectrum filtering technique has been introduced to improve harmonic analysis in this study. The impact of site characterization and physical processes have been examined to justify the necessity of applying the spectrum filtering technique following classic harmonic analysis. Investigations show that the imperfection of the harmonic toolbox, e.g. T_Tide, could be attributed to significant contributions to the long period energy of density gradient and background currents, the greater variability of current amplitude energy, and the larger energy surrounding the clustering diurnal and semi-diurnal tidal constituents. From applications on hydrodynamic model data, this study supports the argument that harmonic analysis with the spectrum filtering technique could significantly improve the accuracy of tidal energy and residuals separation.

Abstract A global search optimization system is applied to the design of a horizontal axis tidal current turbine with shroud. 11 design parameters of the turbine blade and 4 design parameters of the shroud casing are considered for the optimization using a genetic algorithm. In order to reduce the simulation cost, a neural network is applied as the meta-model of the RANS (Reynolds-averaged Navier–Stokes) equation solver. Multi-objectives of the power coefficient at different tip speed ratios are applied to cover a wide operating range of the turbine. The CFD (Computational fluid dynamics) for optimization is validated experimentally for the case of a baseline design, and an optimum design is proposed. In this paper, a static structural analysis has been performed, and its robustness is confirmed under several operating conditions. Furthermore, internal flow of the optimized turbine is discussed in detail. It is found that the optimized blade generates a swirling flow and suppresses flow separation at the diffuser wall. The wide angle of the diffuser successfully achieves a high pressure recovery ratio and results in a high level of suction at the inlet of the turbine. It is found that the high-performance tidal turbine is possible to design if both the blade and the shroud diffuser are optimized at the same time.

Abstract Waves are primarily important for the tidal current power generation as they relate not only to the wave load acting on the tidal turbine but also to the installation/maintenance cost of the device through the workable day rate at the sea. Generally, the tidal current power site is in a narrow strait, and the wave characteristics are strongly dependent on the place. Also, there are few data itself. So, we conducted the wave observation in Naru Strait, Goto City, Nagasaki, where the tidal current power project by the Ministry of Environment is in progress. An AWAC (Acoustic Wave and Current Meter) was deployed on the sea bottom of 20m depth in Naru Strait on 18 May and recovered on 12 November, 2017. The wave data obtained for about 6 months were analyzed as wave spectra and mean directions by the records of surface elevation together with tidal current velocity. In particular, the changes of wave amplitude by the tidal current and the storms including typhoon passing are presented.

Abstract With the long-term goal of developing an ensemble forecast system for coastal flooding, we are developing a dynamically-based, numerical model of the global ocean. The model is based on the NEMO framework and has been used to predict global tides and surges in previous studies. This study focuses on the optimization of the joint prediction of both tides and surges, the two main components of total water level that cause coastal flooding. To improve the predictions of the tide we use a modified form of “spectral nudging”. We show this leads to significant improvements in the prediction of the M 2 tide in the open ocean, and also in the shallow regions closer to shore where the model is not nudged. The median value of the vector difference of the tidal amplitude based on sea level observations and a data-assimilative model, and the predictions of our ocean model, is reduced from 11.2 cm to 2.66 cm by the nudging. The improvement deteriorates significantly however if additional tidal constituents are included in the model (most notably S 2 ). This is explained in terms of spectral leakage between tidal bands associated with the nudging methodology and a straightforward solution is proposed.

Abstract To address the uncertainty in both the monitoring and the forecasting of the environmental conditions for offshore operations, DNV recommends the use of Alpha factors. These factors are defined for the environmental conditions in North Sea and the Norwegian Seas. Since no localised Alpha factors are available for regions outside of North Sea, these factors are conservatively applied everywhere in the world. In a region with benign weather characteristics such as the Mediterranean, this could be overly conservative for the selection of operational weather windows. A deterministic weather forecast is sufficient to assess weather risk for short term operations; nonetheless, uncertainties of longer term predicted weather should still need to be addressed. Shore float-in of umbilicals and cables requires a number of sequential activities with associated durations that build up to a total timeframe. This timeframe is beyond what can be accurately assessed using only deterministic weather forecasts. The operational activities are also very sensitive to tidal, current & sea-state variation; particularly associated with the use of personnel in the water like divers and waders for the removal of floats and the use of small crafts for control of the of the product alignment. The use of Ensemble Forecast which utilises probabilistic approach compared to the deterministic approach, addresses the uncertainty of long term weather forecasting and remove artificially added conservatism such as the Alpha factor. This forecast is derived from multiple simulations, each with minor variations of initial conditions and slightly modified weather models. Ensemble Forecast has been applied in offshore operation planning and execution in a project offshore Egypt in Mediterranean; particularly for a shore float-in of a main umbilical. It was used to define the main operational window, the start of the operation and weather risk beyond deterministic weather forecast. The use of the Ensemble Forecast for selection of overall weather window has been shown to optimise operability, reduce the risk of product being damaged, provide added confidence in the offshore operational safety for the personnel involved while minimising waiting on weather. The strategy of defining storm riding configuration for umbilical could enhanced overall operability. When the operational restrictions relating to limiting activities are anticipated to be surpassed but remains still within the storm riding sea-state, these activities could be safely halted and product held in place in stand-by mode until the weather conditions improved without affecting its integrity or overloading installation equipment.

Abstract The proposed Swansea Bay tidal energy lagoon is an example of a relatively small-scale tidal barrage demonstrator project. A key concern with this technology is that such structures may exacerbate other environmental problems. However, such structures might also create beneficial environmental effects in some areas, such as mitigating the impact of storm surges. In this paper we model the hydrodynamics of the Swansea lagoon and surrounding area using a depth-averaged numerical model. We simulate a number of storm surge events from the past 40 years and analyse how the presence of the Swansea Lagoon (under various operating strategies) modifies the resulting water levels.

Abstract The horizontal-axis tidal current turbine is often installed in near-surface to use the high flow velocity of tidal current, and many designers have found the effect of wave on the hydrodynamic performance of tidal current turbine. The present study focuses on the hydrodynamic analysis of a tidal current turbine in a horizontal axis under the condition of regular waves, based on CFD method. The experimental data are used to verify the feasibility of the method. A non-dimensional parameter k is defined as the ratio of tip submergence to wave amplitude. It is shown that the numerical method is good to predict the hydrodynamic performance of horizontal axis turbine. By comparing the power coefficient and axial load coefficient in different tip submergence and wave amplitude, the effects of tip submergence and wave amplitude on the hydrodynamic performance of tidal current turbine are analyzed.

Abstract Improving the reliability of marine renewable energy devices such as wave and tidal energy convertors is an important task, primarily to minimize the perceived risks and reduce the associated cost for operation and maintenance. Marine systems involve a wide range of uncertainties, due to the complexity of failure mechanism of the marine components, scarcity of data, human interactions and randomness of the sea environment. The fundamental element of a probabilistic risk analysis necessarily needs to rely on operational information and observation data to quantify the performance of the system. However, in reality it is difficult to ascertain observation of the precursor data according to the number of component failures that have occurred, mainly as a result of imprecision in the failure criterion, record keeping, or experimental and physical modelling of the process. Traditional reliability estimation approaches such as Fault Tree, Event Tree and Reliability Block Diagram analysis offer simplified, rarely realistic models of this complex reliability problem. The main reason is that they all rely on accurate prior information as a perquisite for performing reliability assessment. In this paper, a hierarchical Bayesian framework is developed for modelling marine renewable component failures encountered the uncertainty. The proposed approach is capable to incorporate the conditions, which lack reliable observation data (e.g. unknown/uncertain failure rate of a component). The hierarchical Bayesian framework provides a platform for the propagation of uncertainties through the reliability assessment of the system, via Markov Chain Monte Carlo (MCMC) sampling. The advantages of using MCMC sampling has proliferated Bayesian inference for conducting risk and reliability assessment of engineering system. It is able to use hyper-priors to represent prior parameters as a subjective observations for probability estimation of the failure events and enable an updating process for quantitative reasoning of interdependence between parameters. The developed framework will be an assistive tool for a better monitoring of the operation in terms of evaluating performance of marine renewable system under the risk of failure. The paper illustrates the approach using a tidal energy convertor as a case study for estimating components failure rates and representing the uncertainties of system reliability. The paper will be of interest to reliability practitioners and researchers, as well as tidal energy technology and project developers, seeking a more accurate reliability estimation framework.

Abstract In this paper, idealised analytical and numerical models are used to explore the potential for local blockage effects to enhance the performance of turbines in tidal channels. Arrays of turbines modelled using the volume-flux-constrained actuator disc and blade element momentum theories are embedded within one-dimensional analytical and two-dimensional numerical channel domains. The effects of local blockage on the performance of arrays comprising one and five rows of actuator discs and tidal rotors operating in steady and oscillatory channel flow are then examined. In the case of steady flow, numerical results are found to agree very well with the two-scale actuator disc theory of Nishino & Willden [1]. In the case of oscillatory flow, however, numerical results show that the shorter and more highly blocked arrays produce considerably more power than predicted by the one-dimensional two-scale theory. These results support the findings of Bonar et al. [2], who showed that under certain oscillatory flow conditions, the power produced by a partial-width tidal turbine array can be much greater than predicted by two-scale theory. The departure from theory is most noticeable in the case of five turbine rows, where the two-scale theory predicts that the maximum available power should decrease with increasing local blockage but the numerical model shows the maximum available power to increase. The effects of local blockage are found to be less pronounced for the more realistic tidal rotor than for the highly idealised actuator disc but for both models, the results show that in oscillatory flow, considerably more power is available to the shorter and more highly blocked turbine arrays.

Abstract The hydrodynamic performance of a novel current energy generator is studied with consideration of the effect of Wing in Ground (WIG) by Star CCM+. The pitch and heave motions of a turbine with a 2D single oscillating wing and two parallelized oscillating wings in uniform flow are simulated, and the numerical results including the lift force, drag force and moment coefficients of the hydrofoil are calculated to analyze the hydrodynamic performance of the generator. First, the convergence studies with respect to the mesh and time step are firstly carried out by compared with the published data. Secondly, the hydrodynamic performance of the WIG-based current energy extraction is investigated, and a good performance of the current energy extraction is confirmed. Finally, the effect of boundary conditions of wing and wall on the performance of the current energy generator is investigated.

Abstract There are two approaches to extracting power from tides — either turbines are placed in areas of strong flows or turbines are placed in barrages enabling the two sides of the barrage to be closed off and a head to build up across the barrage. Both of these energy extraction approaches will have a significant back effect on the flow, and it is vital that this is correctly modelled in any numerical simulation of tidal hydrodynamics. This paper presents the inclusion of both tidal stream turbines and tidal barrages in the depth-averaged shallow water equation model DG-SWEM. We represent the head loss due to tidal stream turbines as a line discontinuity — thus we consider the turbines, and the energy lost in local wake-mixing behind the turbines, to be a sub-grid scale processes. Our code allows the inclusion of turbine power and thrust coefficients which are dependent on Froude number, turbine blockage, and velocity, but can be obtained from analytical or numerical models as well as experimental data. The barrage model modifies the existing culvert model within the code, replacing the original cross-barrier pipe equations. At the location of this boundary, velocities through sluice gates are calculated according to the orifice equation. For simulating the turbines, a Hill Chart for low head bulb turbines provided by Andritz Hydro is used. We demonstrate the implementations on both idealised geometries where it is straightforward to compare against other models and numerical simulations of real candidate sites for tidal energy in Malaysia and the Bristol Channel.

Abstract The influence of unsteady tidal flow on the flow-induced vibration of a vertical axis tidal turbine blade is investigated numerically in this paper. A 2D CFD model is developed to simulate the blade flow-induced vibration in OpenFoam. The vibration is caused by dynamic loading from the unsteady tide. It is recognized that the unsteady tidal current mainly comes from the changes in tidal velocity magnitude and angle of attack experienced by a tidal turbine blade as it rotates. This paper studies numerically how velocity magnitude and initial angle of attack influence tidal turbine blade vibrations and the effects of the velocity and angle of attack are evaluated separately where the unsteadiness parameters are varied around a set of environmental condition. The vibration is examined through time histories of blade displacement, pressure distribution on the blade surface and the tidal current regime. The blade is assumed to have pitch and heave responses thus the vibration is in the form of transverse and torsional vibrations. The results show that increasing tidal velocity magnitude strengthens the torsional vibration. The increase of angle of attack is likely to generate chaotic motions and enhance both transverse and torsional vibrations.

The U.S. East Coast has the potential for offshore wind deployment and contribute to the energy demands of the area. A safe and cost-effective development of offshore wind farms requires detailed and accurate meteorological and oceanographic (MetOcean) analysis and, thus, a wave and hydrodynamic database has been produced along the U.S. East Coast. The high-resolution wave and hydrodynamic models are calibrated and validated against various NOAA measurements along the U.S. East Coast. The wave model open lateral boundaries are forced by spectral data from a global wave model. Wind forcing for both models were taken from CFSR wind fields, and the hydrodynamic model was also forced by atmospheric pressure field. Spatially and temporally-varying water level and depth-averaged currents were used to force the wave model. The hydrodynamic model was forced by tidal elevations at the boundaries. The model’s resolution varies from 15 km along the offshore boundaries to approximately 3 km near the coastline. Wave spectra resolution was of 25 frequencies and 16 directions. New wave model features are explored, including the effect of atmospheric stability on wave growth, the air-sea density ratio and the current impact on the wave growth rate. These processes are now included during the calibration process of the wave model. Hydrodynamic model results show good agreement with surface elevation measurements, with no bias and small errors. Results of the wave model show overall good results, although some overestimation of the extreme events is noticeable for the studied cases. The high-quality wave and hydrodynamic models developed within this work will serve as a MetOcean database for developers or alternatively as forcing for establishing local high-resolution (100s m resolution) models, i.e. for complex bathymetry at specific wind farm sites. Further work includes longer-term validations, data assimilation and assessment of other processes such as bottom friction and their impact on the estimation of extreme waves.

With widespread market penetration in the offshore sector, synthetic rope materials offer a range of axial compliance that can be exploited for mooring design, ranging from stiff (e.g. high modulus polyethylene) to soft (e.g. nylon). For new applications, such as the marine renewable energy (MRE) sector synthetic ropes are potentially an enabling technology due to their load reduction properties and relative low cost compared to conventional mooring component materials. Existing design, test and certification procedures for ropes were developed for the station keeping of large offshore equipment. Arrays of small, reactive MRE devices will have an entirely different set of mooring load cases with the mooring system potentially an integral part of the power take off system. Larger devices are also likely to be subjected to complex loading as MRE deployment sites are typically highly energetic in terms of wind, wave and/or tidal energy. Consequently devices and associated subsystems are subjected to dynamic load cases in a wide range of complex environmental and operating conditions. Underpinning research is therefore required to understand the behaviour of synthetic materials subjected to highly dynamic loading regimes in terms of operational performance and long-term durability as these factors could have a significant impact on coupled device performance and availability. At present conventional mooring analyses is based on the use of simplified rope characteristics, although several studies including a Joint Industry Project (Syrope) have investigated these aspects. However, at present an industry-standard method to account for the time-varying behaviour of synthetic ropes for offshore equipment moorings does not exist. The SynMaRE (Synthetic ropes for Marine Renewable Energy mooring systems) project aims to develop a time domain analytical model that can adequately represent the time dependent and non-trivial behaviour of synthetic ropes. Instead of being a stand-alone tool, it is intended that the model will be adopted for incorporation in (or with) commercial mooring system software allowing the prediction of mooring loads and device responses to an increased level of accuracy. This paper will present findings from an initial assessment of parameter estimation techniques utilising a simplified viscoelastic and viscoplastic model and validation scenarios based on physical tension-tension test data featuring load cases relevant to MRE mooring systems.

In recent years, tidal current energy has gained wide attention for its abundant resource and environmentally friendly production. This study focuses on analyzing dynamic behavior of a three-bladed vertical axis tidal current turbine. The multibody dynamics code MBDyn is used in the numerical simulation. It performs the integrated simulation and analysis of nonlinear mechanical, aeroelastic, hydraulic and control problems by numerical integration. In this study, tidal current turbine is idealized as an assembly of flexible beams including axis of rotation, arms and blades. We firstly conduct a modal analysis on the tidal current turbine and validate the model with the results obtained by ANSYS. The natural frequencies of blades with different size parameters are compared and the corresponding mode shapes are presented. Next, a parametric study was performed to investigate the effect of internal force on the dynamic response. It is concluded that the proposed method is accurate and efficient for structural analysis of tidal current turbine and this flexible multibody model can be used in the fluid-structure-interaction analysis in the future.